Displacement sensor with an excitation coil and a detection coil

ABSTRACT

To be able to detect a displacement amount of a moving object by a resolution of from micrometer to nanometer by a simple structure. 
     An excitation coil  23  and a detection coil  25  are arranged to align and a magnetic body probe  27  in a conical shape a sectional area of which is change in a longitudinal direction is arranged in a state of being inserted into an air core portion of the excitation coil  23 . An output terminal from the detection coil  25  is connected to an amplifier  33  and a phase shifting circuit  35  is provided between the amplifier  33  and an input terminal of the excitation coil  23 . When the probe  27  of the magnetic body is moved at inside of the air core portion of the excitation coil  23 , inductance of the excitation coil  23  is changed, a phase difference is produced between an input signal inputted to the excitation coil  23  and an output signal outputted from the detection coil  25  and the phase shifting circuit  35  changes a frequency to nullify the phase difference. The displacement amount is detected from the frequency deviation at this occasion.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.10/476,304 filed on Oct. 29, 2003 now U.S. Pat. No. 7,007,554, theentire contents of which are hereby incorporated herein by reference,and in turn claims priority to JP 2001-333299 filed on Oct. 30, 2001.

TECHNICAL FIELD

The present invention relates to a displacement sensor.

BACKGROUND ART

In order to control operation of a microactuator of a manipulator or thelike used in a micromachine or gene manipulation, it is requested thatthe microactuator is small-sized and light-weighed for detecting andmeasuring a displacement amount of a moving object by a resolution offrom micrometer to nanometer. Conventionally, in detecting and measuringa displacement amount by the resolution of from micrometer to nanometer,various systems have been proposed and reduced into practice.Representatively, there can be pointed out laser measurement by a methodof irradiating an object with laser light and detecting a phasedifference brought about in accordance with displacement of the object,a method of using a phenomenon of interference between the laser lightand reference light or the like.

A displacement amount in a nanometer order can be measured by usinglaser measurement technology or the like. However, whereas a size of amicroactuator is in an order of from several centimeters to ten andseveral centimeters, in the laser measurement, a light source, ameasuring portion for detecting and measuring a phase difference orinterference and the like become considerably large-sized. Further, theapparatus becomes expensive since a complicated optical system and ameasuring frequency at a high frequency are used and the like in orderto promote the resolution.

DISCLOSURE OF THE INVENTION

It is an object of the invention to resolve the problem of theconventional technology and to provide a displacement sensor fordetecting and measuring a displacement amount of a moving object by aresolution of from micrometer to nanometer by a simple structure andsuitable for small-sized and light-weighted formation.

In order to achieve the above-described above, a displacement sensoraccording to the invention is characterized in comprising an excitationcoil and a detection coil arranged in a predetermined positionalrelationship, an amplifier an input end of which is connected to anoutput end of the detection coil, a phase shifting circuit providedbetween an output end of the amplifier and an input end of theexcitation coil for shifting to nullify a phase difference by changing afrequency when the phase difference is produced between an inputwaveform inputted to the excitation coil and an output waveformoutputted from the detection coil, frequency measuring means fordetecting a frequency deviation produced by shifting the phase, and arod-like probe of a magnetic body which is inserted into at least one ofair core portions of the excitation coil and the detection coil and asectional area of which is changed in a longitudinal direction fordetecting a displacement amount of an object to be measured from thefrequency deviation produced by displacing the rod-like probe in thelongitudinal axis direction while maintaining a resonating state of aclosed loop including a space between the excitation coil and thedetection coil.

Further, the displacement sensor according to the invention ischaracterized in that the excitation coil is constituted by connectingtwo coils wound such that polarities thereof are directed reverse toeach other in series.

Further, a displacement sensor according to the invention ischaracterized in comprising a light emitting element for making lightincident on an object to be measured, a light receiving element fordetecting a reflected wave from the object to be measured, an amplifieran input end of which is connected to an output end of the lightreceiving element, a phase shifting circuit provided between an outputend of the amplifier and an input end of the light emitting element forshifting to nullify a phase difference by changing a frequency when thephase difference is produced between an input waveform inputted to thelight emitting element and an output waveform outputted from the lightreceiving element, and frequency measuring means for detecting afrequency deviation produced by shifting the phase for detecting adisplacement of the object to be measured from the frequency deviationproduced by displacing the object to be measured while maintaining aresonance of a closed loop including a space between the light emittingelement and the light receiving element.

The displacement sensor according to the invention is constructed by aconstitution in which a detection coil, an amplifier, a phase shiftingcircuit and an excitation coil are connected in this order and arod-like probe of a magnetic body a sectional area of which is changedin a longitudinal axis direction is arranged to be inserted into atleast one of air core portions of the detection coil and the excitationcoil. According to the constitution, when the rod-like probe isdisplaced in the longitudinal axis direction, a frequency deviation isproduced in accordance with a displacement amount while maintaining aresonating state of a closed loop including a space between theexcitation coil and the detection coil and therefore, the displacementamount of the rod-like probe can be detected. Since a frequencydeviation of several kHz is produced by a displacement amount of 1 mm,there can be realized a displacement sensor capable of detecting thedisplacement amount by a resolution of 0.1 through 0.01 micrometer,detecting and measuring a displacement amount of a moving object by aresolution in a micrometer to nanometer order by a simple structure andsuitable for small-sized and light-weighted formation.

Further, the excitation coil is constituted by connecting two coilswound such that polarities thereof are directed reverse to each other inseries. In this case, magnetic fields thereof are canceled by each otherat a vicinity of a point of connecting the two coils and when the twocoils are provided with the same characteristic, a substantiallynullified magnetic field is constituted to balance. By constructing aconstitution of arranging a probe of a magnetic body to be inserted intoan air core portion thereof, a sensitivity with respect to thedisplacement is increased, a frequency deviation equal to or larger than10 kHz is produced by a displacement amount of 1 mm and therefore, therecan be realized a displacement sensor for detecting and measuring thedisplacement amount of the moving object by a resolution at a micrometerlevel by a simple structure and suitable for small-sized andlight-weighted formation.

Further, a displacement sensor according to the invention is constructedby a constitution in which a light emitting element and a lightreceiving element are used, light is made to be incident on an object tobe measured, a reflected wave therefrom is detected and the lightreceiving element, an amplifier, a phase shifting circuit and the lightemitting element are connected in this order. According to theconstitution, when the object to be measured is displaced, a frequencydeviation is produced in accordance with the displacement amount whilemaintaining a resonating state of a closed loop including a spacebetween the light emitting element and the light receiving element andtherefore, the displacement amount of the object to be measured can bedetected. Since a frequency deviation equal to or larger than 10 kHz isproduced by a displacement amount of 1 mm, there can be realized adisplacement sensor for detecting and measuring the displacement amountof the moving object by a resolution in a micrometer through nanometerorder by a simple structure and suitable for small-sized andlight-weighted formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a constitution of a displacement sensor of aninductance directly measuring type according to a first embodiment ofthe invention.

FIG. 2 is a diagram designating the abscissa by a displacement amount ofa probe and designating the ordinate by inductance of a coil and showinga relationship therebetween according to the first embodiment of theinvention.

FIG. 3 is a block diagram of a displacement sensor of an inductance typeaccording to a second embodiment of the invention.

FIG. 4 is a block diagram of a displacement sensor of an inductance typehaving a higher sensitivity according to a third embodiment of theinvention.

FIG. 5 is a block diagram of a displacement sensor of an optical typeaccording to a fourth embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed explanation will be given of embodiments of the invention inreference to the drawings as follows. FIG. 1 is a view showing aconstitution of a displacement sensor 1 of an inductance directlymeasuring type according to a first embodiment. The displacement sensor1 is constituted by a coil 3, a probe 5 of a magnetic body in a conicalshape a sectional area of which is changed in a longitudinal axisdirection, an inductance meter 7 arranged between two terminals of thecoil 3 and a displacement conversion portion 9 for converting a detectedvalue of the inductance meter 7 into a displacement amount. The probe 5of the magnetic body is arranged to be inserted into an air core portionof the coil 3.

According to the constitution, when the probe 5 of the magnetic body isdisplaced at inside of the air core portion of the coil 3 in alongitudinal axis direction (x), inductance of the coil 3 is changed.That is, it is generally known that inductance of a coil having apermeability μ of an air core portion, a sectional area A, a length L ina longitudinal direction and a turn number N is proportional toμ*A*N*N/L. Hence, when the probe 5 of the magnetic body the sectionalarea of which is changed in the longitudinal direction is displaced inthe longitudinal direction (x) at inside of the air core portion of thecoil 3, in accordance with a displacement amount thereof, a volume ofthe probe disposed in the air core portion of the coil 3 is changed, μis substantially changed and the inductance is changed.

FIG. 2 is a diagram designating the abscissa by the displacement amountof the probe 5 and designating the ordinate by the inductance of thecoil 3 and showing a relationship therebetween. Now, it is assumed thata direction of increasing the displacement amount is a direction ofincreasing the volume of the probe 5 of the magnetic body disposed atthe air core portion of the coil 3. In this case, with an increase inthe displacement amount, μ is substantially increased and the inductanceof the coil 3 is also increased. When a rate of changing the sectionalarea of the probe 5 in the longitudinal direction is pertinentlydesigned, a change in the displacement amount and a change in theinductance can be made to be linear.

In this way, the displacement amount of the probe 5 can be provided by asimple structure by displacing the probe 5 of the magnetic body thesectional area of which is changed in the longitudinal direction atinside of the area core portion, detecting the change in the inductanceof the coil 3 at that occasion by the inductance meter 7 and convertinga detected value thereof into the displacement amount by thedisplacement amount conversion portion.

FIG. 3 is a block diagram of a displacement sensor 21 of an inductancetype according to a second embodiment. An excitation coil 23 and adetection coil 25 are arranged by aligning longitudinal axes ofrespective air core portions thereof commonly. Further, a probe 27 in aconical shape a sectional area of which is changed in a longitudinaldirection is provided and the probe 27 is constituted by a magneticbody. Further, the probe 27 is arranged in a state of being insertedinto the air core portion of the excitation coil 23. An output terminalfrom the detection coil 25 and an input terminal to the excitation coil23 are connected to a signal processing portion 31. In the signalprocessing portion 31, the output terminal from the detection coil 25 isconnected to an amplifier 33 and a phase shifting circuit 35 is providedbetween the amplifier 33 and the input terminal of the excitation coil23. A frequency deviation detector 37 is connected to the phase shiftingcircuit 35, further, a displacement amount calculator 39 is connected tothe frequency deviation detector 37.

In this way, by forming a single closed loop resonating circuit byincluding a space between the excitation coil 23 and the detection coil25, that is, a magnetic circuit of the air core portion including theexcitation coil 23—the probe 27—the detection coil 25, supplying energyfrom a power source, not illustrated, and pertinently setting afrequency-gain phase characteristic of the phase shifting circuit 35,resonance can be continued. An inner constitution of the phase shiftingcircuit 35 and operation thereof in such a closed loop resonatingcircuit is described in details in JP-A-9-145691.

In FIG. 3, when the probe 27 of the magnetic body is displaced and movedin the air core portion of the excitation coil 23, since the probe 27 ofthe magnetic body is formed in the conical shape, the volume of themagnetic body in the air core portion is changed. Thereby, as has beenexplained in reference to FIG. 2, the inductance of the excitation coil23 is changed and there is brought about a change in the magneticcircuit of the space between the excitation coil 23 and the detectioncoil 25, that is, the magnetic circuit of the air core portion includingthe excitation coil 23—the probe 27—the detection coil 25. In accordancetherewith, a phase difference is produced between an input signalinputted to the excitation coil 23 and an output signal outputted fromthe detection coil 25 and the phase shifting circuit 35 changes afrequency to nullify the phase difference. A frequency deviation at theoccasion is detected by the frequency deviation detector 37 and thedisplacement amount is outputted by the displacement calculator 39 forprocessing a relationship between the frequency deviation and thedisplacement amount.

A frequency deviation equal to or larger than several 10 kHz is producedby a displacement amount of 1 mm and therefore, the displacement amountcan be detected by a resolution at a micrometer level. Since thefrequency deviation is in an order of several 10 kHz, a processingfrequency of the signal processing portion is comparatively low and thesignal processing portion can be constructed by a simple circuitconstitution.

FIG. 4 shows a displacement sensor 22 of an inductance type having ahigher sensitivity according to a third embodiment. The same notationsare attached to constituent elements common to those of FIG. 3 and anexplanation thereof will be omitted. In this case, there are used coils24 a and 24 b wound such that pluralities thereof are directed reverseto each other and connected in series for an excitation coil 24.According to the constitution, magnetic fields are canceled by eachother at a vicinity of a point of connecting the two coils 24 a and 24 band when the two coils 24 a and 24 b are provided with the samecharacteristic, a substantially nullified magnetic field is constitutedto balance. By constructing a constitution of arranging the probe 27 ofthe magnetic body to insert into the air core portion, a change in thebalance by inserting the probe 27 of the magnetic body can be detectedby a detection coil 26 and a sensitivity with respect to a displacementis further increased.

Since a frequency deviation equal to or larger than 10 kHz is producedby a displacement amount of 1 mm, there can be realized a displacementsensor for detecting and measuring a displacement amount of a movingobject by a resolution at a micrometer level and suitable forsmall-sized and light-welded formation by a simple structure.

A positional relationship between the excitation coil and the detectioncoil may be established by an arranging method by which a constantpositional relationship is fixed by arranging the excitation coil andthe detection coil to align longitudinal axes of respective air coreportions thereof commonly as in FIG. 3, arranging the detection coilconcentrically at an outer periphery of the excitation coil as in FIG. 4or the like. The probe may be arranged by being inserted into at leastone of the air core portions of the excitation coil and the detectioncoil. Other than a conical shape, other shape by which a sectional areathereof is changed such as a portion of a cone, a function body ofrotation constituting an axis of rotation by a longitudinal axisthereof, a portion of a pyramid or the like can be used for the probe.

In this way, there can be realized a displacement sensor for detectingand measuring a displacement amount of a moving object by a resolutionof from micrometer to nanometer and suitable for small-sized andlight-weighted formation by simple constitutions of the excitation coil,the detection coil and the probe and the signal processing portionshaving a comparatively low processing frequency.

FIG. 5 is a block diagram of a displacement sensor 51 of an optical typeaccording to a fourth embodiment. A light emitting element 53 and alight receiving element 55 are provided to be opposed to an object 57 tobe measured. An output terminal from the light receiving element 55 andan input terminal to the light emitting element 53 are connected to thesignal processing portion 31. Constitution and operation of the signalprocessing portion 31 are similar to those of FIG. 3 and therefore, anexplanation thereof will be omitted. In this way, a single closed loopresonating circuit is formed by including a space between the lightemitting element 53 and the light receiving element 55, that is, a pathof light of the light emitting element 53—the object 57 to bemeasured—the light receiving element 55.

In FIG. 5, when the object 57 to be measured is displaced and a changeis brought about at the space between the light emitting element 53 andthe light receiving element 55, that is, in a length of the path of thelight of the light emitting element 53—the object 57 to be measured—thelight receiving element 55, in accordance therewith, a phase differenceis produced between an input signal inputted to the light emittingelement 53 and an output signal outputted from the light receivingelement 55 and the phase shifting circuit 35 changes the frequency tonullify the phase difference. The frequency deviation at this occasionis detected by the frequency deviation detector 37 and the displacementamount is outputted by the displacement amount calculator 39 forprocessing a relationship between the frequency deviation and thedisplacement amount.

Since a frequency deviation equal to or larger than 100 kHz through 1000kHz is produced by a displacement amount of 1 mm, the displacementamount can be detected by a resolution in a nanometer order. Since thefrequency deviation is utilized, a processing frequency of the signalprocessing portion can be produced by a comparatively simple circuitconstitution.

In this way, there can be realized a displacement sensor for detectingand measuring a displacement amount of a moving object by a resolutionof from micrometer to nanometer and suitable for small-sized andlight-weighted formation by simple constitutions of the light emittingelement, the light receiving element and the signal processing portionhaving a comparatively low processing frequency.

INDUSTRIAL APPLICABILITY

The displacement sensor according to the invention can detect andmeasure the displacement amount of the moving object by the resolutionof from micrometer to nanometer by a simple structure and is suitablefor small-sized and light-weighted formation.

1. A displacement sensor comprising: an excitation coil and a detectioncoil arranged in a predetermined positional relationship; an amplifierhaving an input end connected to an output end of the detection coil; aphase shifting circuit provided between an output end of the amplifierand an input end of the excitation coil and configured to provide phaseshifting to nullify a phase difference produced between an inputwaveform inputted to the excitation coil and an output waveformoutputted from the detection coil by changing a frequency when the phasedifference is produced between the input waveform inputted to theexcitation coil and the output waveform outputted from the detectioncoil; frequency measuring means for detecting a frequency deviationproduced by the phase shifting circuit shifting the phase; and arod-like probe of a magnetic body which is inserted into at least one ofair core portions of the excitation coil and the detection coil and asectional area of which is changed in a longitudinal axis direction fordetecting a displacement amount of an object to be measured from thefrequency deviation produced by displacing the rod-like probe in thelongitudinal axis direction while maintaining a resonating state of aclosed loop including a space between the excitation coil and thedetection coil.
 2. The displacement sensor according to claim 1, whereinthe excitation coil comprises two coils connected together in series andwound such that polarities thereof are directed reverse to each other.